A dog clutch of the above kind can be used in many applications, a typical example being in an AWD (All Wheel Drive) vehicle, to which application reference is made in the following.
A drive system of an AWD vehicle may have an engine, a front axle with a differential, and intermediate shaft or cardan shaft, and a rear axle with a differential. In order to control the distribution of the torque not only to the front axle but also to the rear axle in accordance with the driving conditions, an electronically controlled wet disc coupling is arranged in the driveline to the rear axle, often in the intermediate shaft close to the differential.
The function of the coupling when driving the vehicle in an AWD mode is described elsewhere, for example in WO 2011/043722.
When it is desired to drive the AWD vehicle in an FWD (Forward Wheel Drive) mode, the disc coupling is disconnected, i e its discs are separated for preventing them from transmitting any torque. The coupling may be said to be in a disconnect mode. For enhancing this separation effect, the oil normally provided in the coupling for lubricating and cooling its discs can be removed from the coupling. In order to reduce the acceleration of the rotating mass of the intermediate propelling shaft and to eliminate the drag torque in bearings and sealings therefore, a clutch, preferably close to the front axle differential, may be provided to bring the intermediate shaft to a standstill in the FWD mode of the vehicle.
This clutch is preferably a dog clutch with two distinct positions: connected or disconnected. The two coaxial shafts to be connected or disconnected may be provided with end splines, and an axially movable clutch sleeve can be used for the mechanical control of the clutch.
Different patent publications disclose actuators for fast and efficient connection and disconnection of a dog clutch, for example US 2010/0089685. In this design the clutch sleeve is axially transferred by a spring in the direction for connecting the clutch, the direction in which the response requirement is the highest. An electric motor is used for the disconnecting movement and for loading the spring. At an end position a snap mechanism holds the sleeve, and the motor stops. When connection of the clutch is again desired, the motor is further rotated for releasing the snap function, so that the clutch sleeve is free to move axially for connecting the clutch by the spring.
A problem with this technology is that the rotation of the motor has no real connection to the axial movement of the clutch sleeve, and the motor rotation does not provide any information about the position of the clutch sleeve. An important—and in this case lacking—information is whether a correct end position of the sleeve has been reached or splines teeth abut each other, blocking connection of the dog clutch.
For this reason an extra axial position sensor is needed.